High lift devices such as blown wing flaps in aircraft are well known in the aerodynamic arts.
Generally, the conventional type blown wing-flap employs a single narrow opening or slot to blow a sheet of high velocity air tangentially over the upper surface of a deflected flap to energize the flap boundary layer and, with the aid of the well-known Coanda Effect, causes the flow of air to follow the leading edge upper-surface flap contour in a laminar fashion to a reasonable flap angle (about 45.degree.) before local flow separation occurs causing buffeting. The single slot also reduces buffeting intensity due to local flow separation, to a minimum, at normal full flap angles, i.e., angles up to about 60.degree..
A typical example of such a flap is disclosed in U.S. Pat. No. 2,920,844 granted Jan. 12, 1960 to P. L. Marshall et al. However, this type of flap, which employs only a single energizing slot can be deflected only about 60.degree. or less and still maintain relatively tolerable flow.
Other examples of blown flaps such as those disclosed in U.S. Pat. No. 3,259,341 dated July 5, 1966 to R. H. Steidl, and U.S. Pat. No. 3,790,107 dated Feb. 5, 1974 to J. H. Renshaw employ one or more localized nozzle-like devices to blow air over the flap surface but again are severely limited in effectiveness as the flap is deflected to angles greater than 60.degree.. The latter patent appears to be more applicable to a symmetrical control surface, since it shows upper and lower surface blowing slots to improve flow for either direction of deflection.
Thus the severe buffeting encountered in aircraft equipped with standard flaps during large flap deflections is created by local flow separation over the flap. This produces an aft-moving turbulent wake which then traverses the horizontal tail, causing undesirable and unsteady flow over the horizontal tail and/or longitudinal control surface motion which causes the airplane to buffet (shake) appreciably. Under these generally adverse conditions, the primary aerodynamic increment is more drag but with little or no increase in lift, which steepens the approach angle but does little or nothing to reduce approach speed.
It is therefore a desirable objective to substantially reduce or eliminate buffeting and also obtain more lift, by washing or blowing high velocity air over a flap by sequentially uncovering additional spanwise slots as the flap moves down. Therefore, a droop or deflection angle range greater than 60.degree., preferably up to 90.degree., can be used because the gradual step and angle blowing, increases in a cascade effect the volume of high pressure air blown increasing in proportion to the flap angle, thereby preventing flow break-away, from a nominal or minimum amount when the flap is at zero or neutral position to a maximum amount where the flap is at its maximum angle of 90.degree.. Thus a flap angle of 90.degree. can be employed to produce a substantial increase in coefficient of lift, in addition to the drag increase thereby resulting in a substantial decrease in approach and landing speeds with little or no buffeting. By modulating the amount of blowing, at constant flap angle of 80.degree. for example, from zero flow, will result in high drag and low lift. Maximum blowing will result in much higher lift. This air modulation gives the pilot the option of independently (within limits) controlling approach path angle and approach speed. Such a system obviously will enable a naval pilot, for example, to exercise more precise control of high aircraft with respect to touching down at the optimum point on the carrier by permitting last minute corrections to compensate for changing winds and carrier motions in rough seas.
To this end, it is a primary object of my invention to provide a self contained cascade effect blown flap with a minimum of moving parts that is effective over its entire range including 60.degree. to 90.degree. of flap deflection with little or no buffeting and with a consequent appreciable reduction of landing speed which will be determined by the pressure and volume of air fed into the system.
It is a further object of my invention to provide a cascade effect blown flap that blows high velocity air increasing in volume as the flap deflection angle increases.
It is a still further object of my invention to provide a cascade effect blown flap that is relatively simple in construction, reliable in operation, and relatively inexpensive to manufacture.
It is still another object of my invention to provide a cascade effect blown flap that can be applied with equal facility to jet powered commercial as well as military aircraft.
Other objects and advantages inherent in the blown flap will readily be understood in the ensuing detailed description and drawings.